The discussion throughout this specification comes about due to the realisation of the inventors and/or the identification of certain prior art problems by the inventors.
The applicants are aware of a number of transponder systems that provide two dimensional, limited three dimensional or full three dimensional interrogator capabilities. These systems utilise a multiplicity of interrogator coils operating in different coordinate axis, to achieve the resultant two or three dimensional operation.
One example of an interrogator which produces a relatively uniform field in three dimensions is disclosed in U.S. Pat. No. 5,258,766 and international application PCT/AU95/00436. This form of interrogator is known as a Tunnel Reader Programmer (TRP). While a TRP has excellent three dimensional interrogation properties, the inventors have realised that this technology is only suitable for applications where the RFID transponders are moved in and out of the TRP, usually on a conveyor or similar. TRP are inherently unsuitable for applications requiring the interrogator to operate on a flat surface such as a shelf, table or wall. For these applications flat planar antenna coils are used currently.
The inventors have also realised that flat planar antenna coils suffer from producing fields in only one direction at any point relative to the coil and do not have a three dimensional interrogation capability.
The inventors have further realised that when items are stored, for example, on shelving, draws or other means of storage, the orientation of the item, and consequently, the orientation of the tag associated with the item cannot be guaranteed to be in alignment with the direction required for interrogation, especially by a flat planar antenna coil. Thus, if RFID and remote powering is used in applications where orientation of items to be identified cannot be guaranteed, such as shelving and storage systems, document tracking, luggage identification, gaming tokens, by way of example only, the above identified problem can lead to items being missed, that is, not correctly identified.
WO2007030861 discloses an antenna design and method of operation which enables a 3 dimensional interrogation field to be created from a flat planar antenna. In essence, the disclosure of WO2007030861 provides for a series of parallel spaced conductors through which currents are sequentially switched in order to produce both tangential and normal magnetic field components. The spatial relationship of the sequentially switched currents is chosen to ensure that at different times a tangential and a normal magnetic field components are produced at the same location. The conductors are preferably arranged in a planar fashion and the tangential and normal magnetic fields are produced above the planar surface. A single layer of parallel spaced conductors provides for two dimensional operations. Adding a second parallel layer of orthogonally oriented parallel spaced conductors provides three dimensional operations where currents are sequentially switched in both layers.
FIG. 1 illustrates, in schematic form, a conventional single coil of rectangular form through which a current flows. The resulting magnetic field directions are shown and related to the X, Y and Z coordinate directions. At different regions above (or below) the coil, the magnetic field has unique direction which is variously in the X, the Y or the Z directions, or some combination of these directions in transition regions. FIG. 2 illustrates these regions.
FIG. 3 shows an array of coils and illustrates how, when appropriately switched, a field in the X, Y and Z directions is produced as described in WO2007030861. In this regard, by suitably overlapping generally rectangular coils and then sequentially switching each coil so that only one coil is active at any time, at any point above (or below) the overlapped coils, a field in the X direction, the Y direction and the Z direction may be produced at some time. In order to suitably switch the coils as shown in FIG. 3, the signal from an RFID reader must be controlled by a MUX circuit which directs the RFID reader signal to each coil in the array in a sequentially manner. In addition to the MUX, special circuits in each coil are required to tune the coils and ameliorate the effects of coupling (both capacitive and inductive) between coils which can lead to the generation of parasitic currents in the inactive coils. These parasitic currents may cause, amongst other things, distortion of the active coil's magnetic field, changes in the active coil's tuning, increase of the active coil's losses and a reduction of the RFID current in the active coil. These parasitic currents are considered undesirable.
Whilst the inventors are aware that the circuits described in Application WO2009149506 control the switching of the coils in the antenna array and ameliorate the effect of stray coupling, they also may add to the complexity and cost of the antenna array. Where a relatively large array area is required, it is considered that both the cost and complexity of the array may become very high. A relatively high cost of a relatively large area antenna array is considered an impediment which may prevent the implementation of RFID in various applications.
Whilst the inventors are aware of a disclosure in WO2009149506 which shows exemplary circuits for both controlling the switching of the coil array and for ameliorating the effect of stray coupling, there is considered to still be a need for providing improved RFID, especially in storage or shelving applications of RFID. Furthermore, there is a need to create an antenna array that can read in 1, 2 or 3 dimensions over a relatively large area, at a reduced cost. Such an antenna would be considered highly advantageous and allow the wide scale adoption of RFID in applications where the high implementation cost has previously prevented the adoption of RFID.
Throughout this specification the use of the word “inventor” in singular form may be taken as reference to one (singular) inventor or more than one (plural) inventor of the present invention.
It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.